1. Field of the Invention
This invention relates generally to drilling tools and in particular to an apparatus and method for determining the caliper of a well borehole while drilling, tripping or reaming.
2. Description of the Related Art
Many well logging and formation measurement applications require knowledge of the caliper of the borehole. Caliper measurements are typically performed using a wireline caliper tool run into the borehole after tripping the drill string. Caliper measurements while drilling are typically performed using ultrasonic techniques, because a rotating or longitudinally moving drill string poses problems not seen in wireling applications.
Accurate borehole size and geometry information is needed for a large range of applications. Data quality assessment and environmental corrections for formation evaluation (“FE”) sensors require an understanding of the borehole size and geometry at the sensor location. In the wellbore completion process, one needs to know these borehole parameters for placing casing hardware, such as centralizers etc., and for determining an accurate cement volume. Determining regional directional stress from borehole elongation and breakout information and the assessment of suitability of drilling mud system in view of clay swelling or formation filter cake build-up also requires borehole measurement.
Caliper measurements are available from a number of different wireline devices, utilizing either mechanical arms in contact with the borehole wall or acoustic pulse echo sensors. The acoustic pulse-echo methods currently in use are limited in terms of hole size coverage, and in some cases the quality of pulse-echo measurements is degraded due to incompatible return fluid and/or poor formation conditions. With more and more deviated wells being logged with logging while drilling (“LWD”) sensors, an accurate and real-time caliper measurement with a suitable dynamic range is needed.
Wireline tools are known in the art to measure the diameter, also known as the caliper, of a borehole to correct formation measurements that are sensitive to size or standoff. These corrections are necessary for accurate formation evaluation. U.S. Pat. No. 4,407,157 describes a technique for measuring a borehole caliper by incorporating a mechanical apparatus with extending contact arms that are forced against the sidewall of the borehole. This technique has practical limitations. In order to insert the apparatus in the borehole, the drillstring must be removed, resulting in additional cost and downtime for the driller. Such mechanical apparatus are also limited in the range of diameter measurement they provide. Furthermore, these mechanical wireline tools are not suited or a while-drilling environment, because the arms are coupled to the borehole wall when extended. If such a wireline tool were simply incorporated into a while-drilling system, the mechanical arm(s) will be damaged or will break. In some cases, such a tool might damage the borehole wall making any measurement invalid.
Wireline caliper tools are also time-consuming. The drill string must be tripped before running the wireline into the borehole. In view of the excessive time costs in drilling operations, these wireline tests can be quite expensive. Moreover, wireline tools cannot be effectively used in boreholes highly deviated from the vertical, which is often the case in directional drilling.
The typical caliper tools used while drilling today provide the ability to obtain caliper measurements in deviated boreholes. These tools, however suffer from other problems. Ultrasonic tools housed in a tool collar have difficulty when measuring through some borehole fluids. Depending upon the fluid chemistry, viscosity and the presence of particulates, the measurements may be inaccurate or impossible. Furthermore, these highly complex tools are quite expensive and prone to failure during operation in the harsh borehole environment.
There is, therefore, a need for a cost effective while-drilling tool capable of measuring the caliper of a borehole while maintaining high reliability.